It is well known that aralkyl hydroperoxides can be subjected to a cleavage reaction in the presence of an acid catalyst to yield phenols and carbonyl compounds.
This type of reaction is generally applicable to aralkyl hydroperoxides of the form: ##STR1## where: Ar is a benzene nucleus, substituted benzene, alkyl benzene or polynuclear aromatic, R and R' are hydrogen, alkyl groups or cycloalkyl groups, and n is a whole number from 1 to 6.
Numerous methods for performing the catalytic cleavage of aralkyl hydroperoxides have been disclosed in the patent literature. These methods comprise the use of many different catalysts such as sulfuric acid, other strong mineral acids, and Lewis acids, e.g. boron trifluoride, aluminum chloride, etc. For examples of the range of catalysts employed in this cleavage reaction see U.S. Pat. Nos. 3,376,352; 2,718,172; 2,626,281; 3,187,052.
Many of the patents in addition to disclosing catalysts which may be employed, disclose other process improvements for the catalytic cleavage of aralkyl hydroperoxides to produce phenolic compounds. Such improvements are directed to the ends of increasing yield and purity of the phenols produced by this catalytic cleavage of the aralkyl hydroperoxides. The improvements comprise such things as composition of the reaction medium; use of various solvent systems; methods of addition of the reactants to the reaction vessel, and methods of carrying out batch and continuous reactions. These methods of cleaving aralkyl hydroperoxide involve a cleavage reactor which is equipped with a mechanical agitator and a heat exchange means. The heat exchange means is designed to remove the excess heat of reaction and thereby control the reaction temperature. The mechanical agitator ensures contact of the hydroperoxide and catalyst, thus contributing to a relatively complete cleavage reaction. Examples of this type of cleavage reaction, including the equipment used, are given in the following U.S. Pat. Nos.: 3,376,352; 2,663,735; 3,187,052; 3,626,281; and 2,748,172.
The aralkyl hydroperoxides utilized in the acid catalyzed cleavage to form phenols are commonly the products of a direct oxidation of aralkyl hydrocarbons with molecular oxygen. The aralkyl hydroperoxides produced by direct oxidation contain impurities such as unreacted aralkyl hydrocarbon, other oxidation products, water, and alkaline materials.
In the methods where the cleavage catalyst is a strong mineral acid, particularly sulfuric acid, the water and alkaline materials should be substantially removed from the aralkyl hydroperoxide before it is subjected to the cleavage reaction. Water dilutes the acid catalyst and the alkaline materials neutralize the acid catalyst, thus increasing the reaction time and the acid consumption in the cleavage reaction. Water has the additional disadvantage in sulfuric acid catalyzed cleavage reactions that it forms "browning centers" within which highly colored side products are formed. The highly colored side products must be removed so color specifications can be met by the product phenols. In cleavage reactions employing other acid catalysts, such as phosphoric acid for instance, water is not the problem it is in the sulfuric acid systems. The alkaline materials are a problem in all the systems, since they neutralize the catalyst.
The cleavage reactions described above, employing mechanical agitation, have substantially complete backmixing of reaction products with reactants. They also have appreciable residence times of from fifteen minutes to over 2 hours. Such backmixing and long residence times contribute to the level of cleavage reaction side products. These side products, resulting from side reactions within the cleavage reaction are numerous. They vary according to the types of hydroperoxides subjected to cleavage, the catalyst used, and the range of impurities present in the hydroperoxide reactant. In the mechanically agitated reactions which have an appreciable residence time these side reactions can contribute sufficient highly colored side products to cause serious purification problems. Examples of the types of side reactions which can occur in the cleavage reaction are: condensation of the carbonyl compound, such as forming mesityl oxide from acetone; formation of olefins, e.g. divinyl benzene etc; formation of isopropanol phenol; reaction of phenolic products with the carbonyl compound; oxidation of the phenols by the hydroperoxides, such as formation of quinones from hydroquinones; reaction of quinones with hydroquinone to form quinhydrone; and condensation of olefins to form polymers. There are many other side reactions depending upon the reactants present in the cleavage reaction. Many of the side products so formed are highly colored and must be substantially completely removed for the product phenols to meet the color specifications. The patent literature discloses many methods of removing side products and purifying the phenols. For example see U.S. Pat. Nos.: 3,376,352; 3,187,052; 3,043,883; 2,748,172; 2,663,735; 3,140,318; 3,155,734. The purification of phenol products, however, is increasingly difficult at higher concentrations of side products.
Methods have also been devised for avoiding some of the problems associated with such side reactions. These methods generally limit the choice of operating conditions for the cleavage reaction as by determining what solvents or which catalysts may be used. Also, these methods do not have general applicability to cleavage reactions to produce mono and polyhydric phenols, but are limited to particular reactants.
There is a danger of runaway heats of reaction in the backmixed cleavage reaction vessels. The rate of heat release from the cleavage reaction is controlled, under normal conditions, by the rate of hydroperoxide addition. However, various operating condition changes may cause the concentration of hydroperoxide in the cleavage zone to rise to the point where the rate of hydroperoxide addition no longer controls the heat release. Such operating condition changes as acid failure, excess water, excess alkali, and others can cause the hydroperoxide concentration in the cleavage reaction vessel to rise above normal levels. When normal operating conditions are restored, abnormal amounts of heat will be liberated due to the excessive amount of hydroperoxide present in the reaction medium. Unless the heat exchange means is sufficiently large, the vessel temperature will rise causing the cleavage reaction to speed up, thus producing heat at a greater rate, and so on in a cascading fashion. Under these conditions the temperatures and pressures in the cleavage reaction vessel quickly become uncontrollable. The potential hazard to personnel and equipment from such a runaway reaction is apparent.
The prior art indicates that hydroperoxide rearrangement product quality is improved at higher product to reactant ratios in the reaction mixture, and hence continuous, stirred reactors are indicated as preferable for rearranging hydroperoxides to their corresponding phenolic compounds. It has been found, however, as a result of the present invention that product quality can be improved by limiting the contact between the product and the reactants.
An object of the invention is to provide an improved method for the continuous acid catalyzed cleavage of aralkyl hydroperoxides to produce phenols and carbonyl compounds.
Another object of this invention is to provide a method for the continuous acid catalyzed cleavage of aralkyl hydroperoxides to produce phenols and carbonyl compounds wherein the unwanted side reactions associated with the cleavage reaction are held to a minimum.
A further object of the invention is to provide a method for the continuous acid catalyzed cleavage of aralkyl hydroperoxides to produce phenols and carbonyl compounds which may be controlled by varying the reaction variables of temperature, pressure, catalyst concentration and residence time in the reaction zone.
A further object of the invention is to provide a method for continuous acid catalyzed cleavage of aralkyl hydroperoxides which will produce a phenol product of improved purity.
A further object of the invention is to provide a method for the continuous acid catalyzed cleavage of aralkyl hydroperoxides wherein the danger of injury to personnel and equipment occurring during a runaway reaction is greatly minimized or substantially eliminated.
A further object of this invention is to provide a unique reaction chamber wherein the objects above may be realized in relation to the continuous acid catalyzed cleavage of an aralkyl hydroperoxide to produce phenols and carbonyl compounds.
Other objects and a more complete understanding of the present invention may be realized from the following specification and claims when taken in conjunction with the drawing.